Scheduling executors

Earlier in this chapter, we discussed how the command-line runners can be used as a place to start the scheduled executor thread pools in order to run the worker threads in intervals. While that is certainly a possibility, Spring provides you with a more concise configuration to achieve the same goal: @EnableScheduling.

Getting ready

We will enhance our application so that it will print a count of books in our repository every 10 seconds. To achieve this, we will make the necessary modifications to the BookPubApplication and StartupRunner classes.

How to do it…

1. Let's add an @EnableScheduling annotation to the BookPubApplication class, as follows:

   @SpringBootApplication
   @EnableScheduling
   public class BookPubApplication {…}

2. As an @Scheduled annotation can be placed only on methods without arguments, let's add a new run() method to the StartupRunner class and annotate it with the @Scheduled annotation, as shown in the following line:

   @Scheduled(initialDelay = 1000, fixedRate = 10000)
   public void run() {
     logger.info("Number of books: " + bookRepository.count());
   }

3. Start the application by executing ./gradlew clean bootRun from the command line so as to observe the Number of books: 0 message that shows in the logs every 10 seconds.

How it works…

Like some other annotations that we discussed in this chapter and will further discuss in the book, @EnableScheduling is not a Spring Boot annotation, but instead is a Spring Context module annotation. Similar to the @SpringBootApplication and @EnableAutoConfiguration annotations, this is a meta-annotation and internally imports the SchedulingConfiguration via the @Import(SchedulingConfiguration.class) instruction, which can be seen if looked found inside the code for the @EnableScheduling annotation class.

ScheduledAnnotationBeanPostProcessor that will be created by the imported configuration will scan the declared Spring Beans for the presence of the @Scheduled annotations. For every annotated method without arguments, the appropriate executor thread pool will be created. It will manage the scheduled invocation of the annotated method.